1. Field of the Invention
The present invention relates to a thin film optical element and a method for producing the same.
2. Description of the Prior Art
Various researches have been conducted for applying thin film optical elements, i.e. optical elements utilizing thin film light guide, to light deflector, light modulator, spectrum analyzer, light switch, etc. In such a thin film optical element, the refractive index of the light guide is varied by an external effect such as an acousto-optical effect or an electro-optical effect to modulate or deflect the light propagating in said light guide. The substrate for such optical element is often composed of lithium niobate (LiNbO.sub.3) crystal or lithium tantalate (LiTaO.sub.3) which shows excellent piezoelectric, acousto-optical and electro-optical effects and a low light propagation loss. A representative method of producing a thin film light guide with such crystalline substrate consists of thermally diffusing titanium (Ti), at a high temperature, in the surface of said crystalline substrate, thereby forming a light guide layer of a refractive index slightly higher than that of said substrate. However, the thin film light guide thus prepared often suffers from optical damage, thus only allowing entry of light of a very small power. The optical damage means a phenomenon in which the intensity of light taken out from the light guide after propagation therein does not increase proportionally, because of scattering, with the intensity of light entering said light guide.
As another method for avoiding such optical damage, there is known ion exchange method in which a crystalline substrate for example of LiNbO.sub.3 or LiTaO.sub.3 is subjected to a low-temperature thermal treatment in a fused salt such as thalium nitrate (TlNO.sub.3), silver nitrate (AgNO.sub.3) or Potassium nitrate (KNO.sub.3) or in a weak acid such as benzoic acid (C.sub.6 H.sub.5 COOH) to exchange lithium ion (Li.sub.+) in said crystalline substrate with ion such as proton (H.sub.+) of weak acid thereby obtaining a light guide with a large difference in the refractive index (.DELTA.h:0.12). The threshold value of the optical damage of the thin film light guide prepared with such ion exchange method is several tens of times higher than that obtained by titanium diffusion, but said ion exchange treatment deteriorates the piezoelectric property and electro-optical effect specific to the crystal of LiNbO.sub.3 or LiTaO.sub.3, thus reducing the diffraction efficiency in the use as light deflector.
In the formation of a light deflector or a light modulator utilizing acousto-optical or electro-optical effect, it is essential to improve the efficiency of such effect. A representative example utilizing acousto-optical effect consists of applying a high frequency electric field between comb electrodes formed by photolithography on a light guide, thus inducing a surface elastic wave on said light guide. In such structure, it is already known that the interaction between the surface elastic wave induced on the light guide and the light guided in said light guide becomes larger if the energy distribution of the guided light is enclosed in the vicinity of the surface of the substrate [C. S. Tsai, IEEE Transactions on Circuits and Systems, Vol. Cas.-26, 12, 1979] .
On the other hand, the input or output of light between a semiconductor laser or an optical fiber and such light guide is conducted through the end face thereof, so that the energy distribution of the guided light has to be broadened in the thickness direction of the substrate, corresponding to the energy distribution in the optical fiber or the like, in order to improve the efficiency of optical connection.
Consequently conventional thin film optical element has been unable to achieve a high efficiency in modulation or deflection and a high efficiency in optical connection at the same time, because the required energy distribution of the guided light is different between the optical connector part for input/output of light and the optical functional part for modulation or deflection of the guided light. Also in order to resolve this problem there has been proposed a method, in case of forming the light guide by titanium diffusion, by differentiating the concentration of titanium diffusion between the optical connector part and the optical functional part [M. Kondo, K. Komatsu & Y. Ohta; Society of Applied Physics 84 Spring Symposium, preprint 31a-K-7; and ibid.; 7th Topical Meeting on Integrated and GuidedWave Optics TuA5-1]. However an effective method for avoiding the above-mentioned problem has not been known in the case where the light guide is formed by the aforementioned ion injection method.